In magneto-optic recording, data is represented by a magnetized domain, which is a stable magnetizable region having a preferential magnetized orientation. The preferentially magnetized domain is often referred to as a "bit." The preferential magnetized orientation is caused by an energy source, such as a laser, of sufficient intensity heating the medium above its Curie temperature while the medium is simultaneously biased by a magnetic field. The laser is used to heat the magnetic medium in the localized area. When the localized area exceeds the Curie temperature, the magnetization direction is set by the magnetic field.
When the laser beam is removed, the bit cools in the presence of the biasing magnetic field and has its magnetization switched to that direction. The momentary temperature rise in the bit reduces the bit's coercive force so that the magnetic biasing field forces the magnetization of the domain to conform to the direction of the biasing magnetic field as the bit cools below its Curie temperature.
The never-ending tendency in magneto-optic recording is toward higher storage capacities. However, the amount of information that can be stored on a magneto-optic disc is limited by the degree to which the size of a focused laser beam spot can be decreased. One method of reducing the effective area of the laser beam spot is to take advantage of the fact that when a laser is focused to a spot on a moving medium, the temperature of the medium under the spot will not be uniform. If the medium is comprised of a material having a transition temperature within the range of temperatures generated in the medium under the spot, it is possible to erase data recorded in one portion of the spot while reading information in another portion. More specifically, data is erased from a readout layer in the medium at a higher temperature (i.e., at the rear of the beam spot) while subjecting that portion of the medium to an external magnetic field, and data is read from the read out layer at a lower temperature (i.e., at the front of the beam spot).